Abstract: The significant variation of seasonal streamflow in mountainous regions prevents the existing irrigation channel of the water volume facilities from meeting the flow measurement accuracy requirements in an extensive flow range. In this paper, a combined water measuring facility based on crump weir and sluicing flume is proposed, and the mechanism of its flow measurement is explored in an extensive flow range. Based on the hydraulic performance test of the generalized flume in the range of 5-79 L/s, the changes of the water surface line, Froude number (Fr), vertical longitudinal average velocity, and the characteristic length and width of a thin water layer were analyzed under different flow rates. Furthermore, the flow measurement equations were formulated in different flow threshold ranges. The results show that: ① As the flow rate increases, the flow pattern of the combined facility changes from being an in-groove flow to a weir flow, and the relative water depth of the threshold value corresponding to the flow rate is 0.885. The flow measurement formulas of the in-groove flow and the weir flow are obtained through a fit. The relative error is less than 3% compared with the measured flow rate. ② The in-groove and the weir flow have different characteristics in the combined facility; when the in-groove flow occurs, the average longitudinal velocity, the characteristic length and width of the thin water layer, and the comprehensive flow coefficient of each measuring point increases with the flow rate. When the weir flow occurs, the average longitudinal velocity of each measuring point in the front of the flume decreases as the flow rate increases. In contrast, the average longitudinal velocity of each measuring point in the back increases with the flow rate. The average section velocity near the middle of the contraction torsion surface in the flume is the same. ③ The combined facility flow coefficient m₀ decreases as the discharge increases. ④ The characteristic length and width of the thin water layer downstream decrease as the flow rate increases, and the maximum value is reached at the flow threshold. This study effectively solves the lack of flow measuring facilities in open channels with a significant flow fluctuation. It can provide a reference in the application of flow measuring facilities for seasonal streams in mountainous regions.